Dynamic loudspeakers are generally used to produce audio output from electrical signals, and such dynamic loudspeakers consist of a diaphragm in the shape of a cone driven by a voice coil located in a magnetic field. The factors which influence the performance of conventional dynamic loudspeakers are well-known as indicated by chapter 6 of "Acoustical Engineering" by Harry F. Olson, D. Van Nostrand Company, Inc. 1957.
One of the factors which influences the performance of a dynamic loudspeaker is the uniformity of the magnetic field as seen by the voice coil during its excursion. In practice, the magnetic field is established in a gap between a cylindrical opening in a flat ferromagnetic plate, referred to as a front plate, and a cylindrical ferromagnetic pole piece mounted co-axially within the opening of the plate. The voice coil is mounted on the diaphragm for translation along the axis of the pole piece, and the length of the voice coil relative to the thickness of the plate is a factor in determining the uniformity of the flux experienced by the voice coil during its excursion. To subject the voice coil to uniform flux, the voice coil is often made longer than the magnetic gap, so that approximately the same number of turns of the voice coil are disposed within the magnetic gap regardless of the deflection of the diaphragm.
The use of a voice coil, which is long compared to the thickness of the plate which forms the magnetic gap, however, causes the ends of the voice coil to experience increased temperature operation. The resistance of the voice coil reacts with the current flowing through the voice coil to generate heat uniformly throughout the entire length of the voice coil. Since the voice coil is operating in an air gap, the principal mechanism for heat removal is convection and the principal heat flow from the voice coil is to the portion of the front plate and pole piece which confront the magnetic gap. Heat conduction from the pole piece and front plate to the perimeter of the loudspeaker frame maintains these elements relatively cool, since heat conduction is a much more effective way for heat removal than convection.
The overhanging upper and lower portions of the voice coil are cooled by convection over relatively long pathways unless another mechanism is provided to cool these portions of the voice coil. Heating of the voice coil is undesirable, since it raises the resistance of the voice coil and creates thermal power compression. Long-term heating of the voice coil reduces the overall efficiency of the diaphragm driver which may degrade performance. Also, the ability of the loudspeaker to react to input transients may be degraded resulting in thermal clipping.
Offenlegungsschrift No. 2,605,613 published by the Republic of Germany on Feb. 9, 1976 entitled LAUTSPRECHER describes a loudspeaker construction in which the overhanging portions of the voice coil, both above and below the gap, are cooled by non-magnetic blocks mounted above and below the front plate of the loudspeaker, the blocks extending the outer wall of the gap on both sides of the front plate. The Offenlegungsschrift also describes a cylindrical block mounted on the central pole piece which extends the inner wall of the magnetic gap outwardly from the central pole piece. This constructions shortens the path of convection heat transfer from the upper and lower overhang portions of the voice coil, and provides a conduction path for heat from these portions of the voice coil coupled to the shorter convection path.
Dynamic loudspeakers ultimately depend upon transfer of heat from the voice coil to the ambient atmosphere to limit temperature, and this transfer occurs principally by convection from the frame and magnetic structure of the loudspeaker to the surrounding air. Further, the thermal resistance between the front plate and the frame of the loudspeaker is relatively low because heat is transferred principally by conduction, and transfer of heat from the frame t the surrounding air becomes the ultimate limitation. It is an object of the present invention to improve the cooling which can be achieved for the voice coil over that of prior constructions, and particularly the transfer of heat to the surrounding air.
In all dynamic loudspeakers, it is undesirable to permit the voice coil to scrape against either the inner or outer wall of the magnetic gap, since scraping will deteriorate the voice coil and will produce distortion in the sound output. The conventional construction to maintain the voice coil at a distance from the walls of the gap and cylindrical with the central pole piece is to mount the coil on a coil former or bobbin, which is cemented to the diaphragm, and to utilize a flexible surround at the perimeter of the diaphragm and a spider mounted between the coil former and the frame to restrict movement of the coil former to translation along the central axis of the pole piece. As the power of loudspeakers increases, however, the force exerted on the coil increases, and it is necessary for the coil to be translated over longer and longer distances to reproduce low frequencies. These factors have made it difficult to avoid contact between the voice coil and the walls of the gap, and efforts have been made to ameliorate the undesirable effects of contact between the voice coil and the walls of the gap. U.S. Pat. No. 4,547,632 of Michael Bryson entitled DYNAMIC LOUDSPEAKER seeks to minimize the effects of contact of the voice coil with the walls of the gap by coating the walls of the gap with a plastic material. It is a further object of the present invention to provide a dynamic loudspeaker with a voice coil mounted on a loudspeaker diaphragm and maintained centrally of the magnetic gap by an improved and novel spider mechanism which is suitable for high power use and overcomes these adverse effects.
Efforts have been made to improve the suspension of the voice coil mechanism of a loudspeaker as disclosed in U.S. Pat. No. 4,387,275 of Yasuomi Shimada, Tatsuo Fukuyama, and Toshiyuki Mizutani dated June 7, 1983 and entitled SPEAKER AND SPEAKER SYSTEM. In a specialized type of loudspeaker, two or more spiders are employed between the coil former and the frame or the cone of a loudspeaker. U.S. Pat. No. 4,379,952 of Kaizer, et al. entitled MECHANICAL FILTER FOR AN ELECTRODYNAMIC TRANSDUCER also has an embodiment with two centering disks between the voice coil and the frame in another specialized loudspeaker. The Handbook for Sound Engineers, Glen Ballou, Editor, 1987, Howard W. Sams & Company, illustrates at page 430 a Gauss cone loudspeaker utilizing a double spider between the frame and the coil former. The Handbook for Sound Engineers, supra, also discloses a Stroker loudspeaker of Cerwin-Vega in which a spider extends between the coil former and frame, and a second spider-type disk is mounted between a post extending from the pole piece of the loudspeaker and a portion of the diaphragm of the loudspeaker spaced from the coil former.
It is an object of the present invention to provide a loudspeaker with a diaphragm and voice coil mounted on a coil former with a voice coil suspension superior to that of prior loudspeakers, particularly a suspension that permits extended axial travel of the voice coil without scraping of the gap structure.